Assessment of antemortem tests used in the control of an outbreak of tuberculosis in llamas (Lama glama)

Abstract

An outbreak of tuberculosis (TB) caused by Mycobacterium bovis in a llama herd is described. Over a 25-month period, a total of 70 llamas were selected for postmortem examination using four distinct criteria: clinical suspicion of disease (15 animals), positive tuberculin skin test result (three animals), antibody positive using a novel serological test (Rapid Test, 54 animals) and elective cull (five animals). Some animals qualified on more than one criterion. Gross lesions of TB were detected in 15 animals, with lung and lymph node lesions consistently observed. Samples were collected from 14 of 15 animals with visible lesions as well as those with no visible lesions, for histopathology and mycobacterial culture. All 14 llamas with visible lesions had caseonecrotic granulomatous lesions associated with acid-fast bacteria and variable mineralisation, and M bovis was isolated from 13. There were no histopathological lesions of TB in llamas with no grossly visible lesions, and M bovis was not isolated from any of these. The predictive value of suspicious gross lesions at postmortem examination was therefore high in the herd. Molecular typing results indicated that the outbreak was caused by a single strain likely to have originated from a local reservoir, probably cattle or wildlife. Antemortem indicators of infection assisted control of the outbreak, but no single test accurately identified all TB cases. Visible lesions were detected in nine of 15 llamas with clinical suspicion of disease, in two of three that had positive tuberculin skin test results and in 10 of 54 that were antibody positive; there was none (zero out of five) in llamas that were electively culled.

Zoonotic diseases in South American camelids in England and Wales

“…[10] Of the 25 human cases of M. microti recorded worldwide in the published literature, nine were of the spoligotype previously isolated from llamas (although this does not necessarily indicate that llamas are a reservoir host). Only one of the cases (a 44 year old woman from Scotland) was known to have had a possible connection to SAC.…”

“…[9] Tuberculosis The majority of TB cases identified in SAC in England and Wales are due to Mycobacterium bovis, however Mycobacterium microti has also been described. [10] Both M. bovis and M. microti can cause illness in humans that is virtually indistinguishable from that caused by M. tuberculosis. Symptoms in humans may include a fever, persistent cough, blood in sputum and weight loss.…”

“…Others occur as sudden deaths. [10] Most cases of TB in SAC are detected following a diagnostic postmortem examination. Although infection can be suspected on the basis of clinical disease, tuberculin testing or serological testing, none of these is completely reliable in identifying individual TB cases.…”

“…Although infection can be suspected on the basis of clinical disease, tuberculin testing or serological testing, none of these is completely reliable in identifying individual TB cases. [10] Under the Tuberculosis (Deer and Camelid) (England) Order 2014 (and similar orders made by the devolved governments), the suspicion of TB in the carcase of a farmed or pet mammal is notifiable to APHA. Since the introduction of these Orders, there has been an increase in the number of samples/carcases received by APHA for examination and TB culture.…”

“…Since the introduction of these Orders, there has been an increase in the number of samples/carcases received by APHA for examination and TB culture. [10] Mycobacterium bovis Between 2000 and 2015, bacteriologically confirmed M. bovis infection was reported in 98 camelid herds in England and Wales [16] Nearly all of those cases occurred in areas where TB is endemic in cattle and badgers, and it is possible that cases in SAC reflect infection spillover from those true reservoir species. There is also some evidence from field epidemiological investigations undertaken by APHA to suggest that TB can additionally spread between SAC premises through the movement of undetected infected camelids.…”